implement clearCache() in DistributeRamCache by using different keys in low-level.
clear all caches before running test, mostly for distributed ram cache.
use distributed ram cache in all cache factories.
update volatile memcached connection string when test instance is generated or loaded.
the revision value in memcached should be kept as long as possible.
store revision cache in getRevision() if not yet exists.
fixup 1061827d6e. delete() and has_key() should also consider revision.
explicitly setup Ram Cache in test_02_RamSession.